Automated material handling systems are used in a variety of industries to move various materials from one location to a another location. Semiconductor fabrication facilities, in particular, commonly employ automated material handling systems for fabricating integrated circuits on semiconductor wafers.
A conventional semiconductor fabrication plant typically includes multiple fabrication areas or bays interconnected by a path, such as a conveyor belt. Each bay generally includes the requisite fabrication tools (interconnected by a subpath) to process semiconductor wafers for a particular purpose, such as photolithography, chemical-mechanical polishing, or chemical vapor deposition, for example. Material stockers or stocking tools generally lie about the plant and store semiconductor wafers waiting to be processed. The wafers are usually stored in cassettes each of which typically hold up to 25 wafers. Each material stocker typically services two or more bays and can hold hundreds of cassettes.
The semiconductor fabrication plant, including the bays, material stockers and the interconnecting path, typically operates under control of a distributed computer system running a factory management program, such as WorkStream Open sold by Consilium, Inc. In this environment, the automated material handling system may conceptually include the cassettes, the transportation system (e.g., paths) and control system (e.g., the distributed computer system).
A typical semiconductor fabrication plant, such as the one described above, is capable of processing thousands of wafers at any given time. The wafers are typically divided into lots which undergo different processing sequences. Each processing sequence typically includes a number of processing steps, each defined by a process specification. In order to manage the transportation required for processing such a large number of wafers through various processing steps, manufacturers commonly employ transfer systems. The efficiency of the transfer systems is important for maintaining the efficiency of the overall manufacturing process and keeping the manufacturing cost of the wafers at as low as possible. Many present material handling systems, however, exhibit excess movement of materials through the system which negatively impacts manufacturing throughput.
Semiconductor manufacturers compete in a highly competitive and capital-intensive industry. A state-of-the-art semiconductor fabrication plant typically includes hundreds of different fabrication tools and can cost $1 billion or more. New plants can also become obsolete relatively quickly as the dimensions of semiconductor devices decrease. Consequently, to manufacture a cost-effective competitive product, semiconductor manufacturers continually seek to increase the throughput and yield of semiconductor wafers.